Prepare PlainLink for Show HN testing

[HexCodeYT]

What changed

• Reworks the repository README into the Show HN landing page with a demo, real before/after URLs, direct GitHub Release installation steps, Gatekeeper instructions, privacy guarantees, and a clear technical-preview warning.
• Adds the HN submission title and opening comment to the launch checklist.
• Restores the demo GIF/MP4 and tester-focused issue templates omitted from the prior merge.
• Ad-hoc signs and verifies preview app bundles before packaging.
• Fixes the icon fallback so it retains multiple resolutions instead of producing a 32 px-only ICNS.
• Adds Preview 3 release notes and corrects signing terminology throughout the documentation.

Why

The app is technically ready for testers, but the remote repository did not provide an easy installation path and the Releases page had no downloadable build. The previous icon fallback also silently reduced the packaged icon to one small representation when iconutil rejected the generated iconset.

Impact

The GitHub repository can serve directly as the Show HN URL. Technical testers get accurate installation expectations, and Preview 3 packaging now produces an ad-hoc-signed, verifiable app bundle with a multi-resolution icon.

Verification

• cargo fmt --check
• cargo test
• cargo clippy --all-targets -- -D warnings
• scripts/test-macos-app.sh
• PLAINLINK_RELEASE_VERSION=v0.1.0-preview.3 scripts/package-macos-app.sh
• shasum -a 256 -c PlainLink-0.1.0-preview.3-macos-arm64.zip.sha256

All checks passed on macOS arm64.